It's not about public safety... it's about revenue! Issuing traffic tickets is a way for local government to generate revenue. Interfering with that by warning other drivers about police causes a drop in revenue, which, first amendment be damned, has to be avoided at all costs.
Funny... or TRAGIC! Didn't we already go down this road already with TORTURE?! Other countries torture, so it's OK if we do it?! Where did that get us? Haven't we brought America down to everyone else's level enough? It's time to take the high ground -- it used to be when I travelled outside the US, I was proud of my country! We have to get that feeling back!